Acid/salt separation

ABSTRACT

The invention provides a method for preparing a carboxylic acid, which method includes the steps of providing magnesium carboxylate, wherein the carboxylic acid corresponding with the carboxylate has a solubility in water at 20° C. of 80 g/100 g water or less; acidifying the magnesium carboxylate with HCl, thereby obtaining a solution comprising carboxylic acid and magnesium chloride (MgCl 2 ); optionally a concentration step, wherein the solution comprising carboxylic acid and MgCl 2  is concentrated; precipitating the carboxylic acid from the solution comprising the carboxylic acid and MgCl 2 , thereby obtaining a carboxylic acid precipitate and a MgCl 2  solution.

This is a Division of application Ser. No. 14/238,686 filed Jul. 28,2014, which in turn is a National Stage application of PCT/NL2012/050574filed Aug. 16, 2012, which claims the benefit of U.S. ProvisionalApplication No. 61/524,353 filed Aug. 17, 2011 and European PatentApplication No. 11177633.2 filed Aug. 16, 2011. The disclosure of theprior applications is hereby incorporated by reference herein in itsentirety.

The invention is directed to a method for preparing a carboxylic acid.The production of carboxylic acids leads to various unwanted byproducts,especially when produced by means of fermentation. Fermentationprocesses wherein carboxylic acids are excreted by the micro-organismswill result in a decrease in the pH. Since such a decrease in pH candamage the micro-organism's metabolic process, it is common practice toadd a base in the fermentation media in order to neutralize the pH. As aresult, carboxylic acid produced in the fermentation media is typicallypresent in the form of a carboxylic acid salt.

A disadvantage of obtaining the carboxylic acid from a fermentationprocess in the form of a carboxylic acid salt is that one or moreadditional steps are required to separate the carboxylic acid from thesalt, i.e. convert the salt to a carboxylic acid, which typically leadsto loss of carboxylic acid and/or carboxylic acid salts and thus to adecrease in the total fermentation or process yield.

A further disadvantage of such steps is that these typically lead toconsiderable salt waste. For example, the separation steps oftencomprise acidulation of the carboxylic acid salt using sulphuric acid,resulting in a sulphate salt as a waste product.

An object of the invention is to provide a separation step in which thecarboxylic acid is separated from a salt solution with a suitableconversion yield.

A further object of the invention is to provide a method with no orsubstantially no salt waste.

The present invention also provides a very robust method which iscapable of separating the targeted carboxylic acid from salt solutionswith of significant low quality due to the presence

At least one of these objects was met by providing a method forpreparing a carboxylic acid, which method comprises the steps of

-   -   providing magnesium carboxylate, wherein the carboxylic acid        corresponding with the carboxylate has a solubility in water at        20° C. of 80 g/100 g water or less,    -   acidifying the magnesium carboxylate with hydrogen chloride        (HCl), e.g. hydrochloric acid, thereby obtaining a solution        comprising carboxylic acid and magnesium chloride (MgCl₂);    -   optionally a concentration step, wherein the solution comprising        carboxylic acid and MgCl₂ is concentrated;    -   precipitating the carboxylic acid from the solution comprising        the carboxylic acid and MgCl₂, thereby obtaining a carboxylic        acid precipitate and a MgCl₂ solution.

The inventors found that the addition of HCl to a magnesium salt of theselected carboxylic acids and subsequent precipitation of the carboxylicacid from the solution leads to a very efficient isolation of thecarboxylic acid from said magnesium carboxylate solution.

In particular, it was found that carboxylic acid could be precipitatedfrom a carboxylate solution acidified with HCl with a very highefficiency. Without wishing to be bound by any theory, the inventorsexpect that the high efficiency of the precipitation is due to aparticular high salting out effect of MgCl₂ in the solution. Inparticular, the salting out effect is expected to be caused by thespecific combination of HCl, magnesium and carboxylic acid. Sincesalting out effects are generally hard to predict, the particular highsalting out effect for these acids observed in the method of theinvention came as a surprise to the inventors.

Thus, using the method of the invention, a carboxylic acid precipitatecan be obtained in a high yield from a magnesium carboxylate solution,which solution is for example a fermentation mixture obtained in afermentation process. Furthermore, the obtained carboxylic acidprecipitate has a relatively high purity, since the precipitation stepin the method of the invention does not result in precipitation of largeamounts of compounds other than carboxylic acid. Furthermore, amagnesium chloride solution is obtained. This solution can be processedfurther as described below.

Furthermore, the specific choice for HCl and magnesium carboxylateprovides for a reduction in salt waste, in particular when combined witha thermal decomposition step.

Preferably, the method further comprises the steps of

-   -   subjecting the MgCl₂ solution to a thermal decomposition step at        temperatures of at least 300° C., thereby decomposing the MgCl₂        to magnesium oxide (MgO) and HCl; and    -   optionally dissolving the HCl formed in the thermal        decomposition step in water, thereby obtaining a HCl solution;        and    -   optionally bringing the MgO in contact with water, thereby        obtaining Mg(OH)₂, which Mg(OH)₂ solution is optionally recycled        for use in a fermentation process, preferably the fermentation        process with which the magnesium carboxylate from the first step        is provided.

The advantage of these additional steps is that a method may be obtainedthat has no or substantially no salt waste. The HCl solution may berecycled to the acidulation step of the method of the invention. TheMg(OH)₂ can be recycled for use in the fermentation process.

The term “carboxylate” as used herein refers to the conjugate base of acarboxylic acid, which generally can be represented by the formulaRCOO—. The term “magnesium carboxylate” refers to the magnesium salt ofthe carboxylic acid to be prepared in the method of the invention.

The term “carboxylic acid corresponding with the carboxylate” refers tothe carboxylic acid that can be obtained by acidifying the carboxylate.This is also the carboxylic acid that is the product the method of theinvention. It may therefore also be referred to as acidifiedcarboxylate. The carboxylic acid corresponding with the carboxylate cangenerally be represented by the formula RCOOH.

The term “precipitating” as used herein refers to the formation of solidmaterial starting from a fully dissolved state. Carboxylic acid can beprecipitated in crystalline form or in amorphous form. By precipitatingcarboxylic acid according to the method of the invention, the carboxylicacid may also be purified. In case the magnesium carboxylate solutioncomprises dissolved impurities, precipitation of carboxylic acidtypically separates the carboxylic acid from such impurities.

The term “solution to be precipitated” as used herein refers to thesolution that is to be subjected to precipitation. Typically, this termrefers to the solution comprising carboxylic acid and MgCl₂ obtainedafter acidulation, optionally after this solution has been subjected toa concentration step and/or a step wherein extra MgCl₂ is added.However, in case of a second or further precipitation step, the term“solution to be precipitated” refers to the MgCl₂ solution obtainedafter the final or latest precipitation step, optionally after thissolution has been subjected to a concentration step and/or a stepwherein extra MgCl₂ is added. Such MgCl₂ solutions may still comprisecarboxylic acid, which may be obtained by subjecting it to a second orfurther precipitation step.

Any magnesium carboxylate can be used, which in acidified form (i.e.wherein the corresponding carboxylic acid) has a solubility in waterclose to or lower than MgCl₂. Consequently, the carboxylic acid to beprecipitated in the method of the invention has a solubility in water of80 g/100 g water or less at 20° C. Carboxylic acids having a solubilityin water considerably higher than MgCl₂ are not suitable to beprecipitated with the method of the invention, because in this caselarge amounts of MgCl₂ will precipitate when precipitating thecarboxylic acid, such that no suitable separation is obtained.

Preferably, the carboxylic acid corresponding with the carboxylate has asolubility that is lower than that of MgCl₂, as measured in water at 20°C., i.e. has a solubility in water of less than 54.5 g/100 g water at20° C. (anhydrate). More preferably, the carboxylic acid has asolubility that is considerably lower than MgCl₂, such that MgCl₂ doesnot precipitate together with the carboxylic acid from the solution inthe precipitation step. Therefore, the carboxylic acid preferably has asolubility in water at 20° C. of less than 60 g/100 water, morepreferably less than 50 g/100 g water, even more preferably less than 40g/100 g water, even more preferably less than 30 g/100 g water, evenmore preferably less than 10 g/100 g water, even more preferably lessthan 7 g/100 g water. The lower boundary for the solubility of thecarboxyic acid is not critical.

The carboxylic acid to be prepared by the method of the invention may beselected from the group consisting of succinic acid, adipic acid,itaconic acid, 2,5-furandicarboxylic acid, fumaric acid, citric acid,maleic acid, glutaric acid, malonic acid, oxalic acid and fatty acidshaving more than 10 carbon atoms. Good results have been obtained usinga carboxylic acid selected from the group consisting of adipic acid,itaconic acid, 2,5-furandicarboxylic acid and fumaric acid. In oneembodiment, the carboxylic acid is not succinic acid.

The magnesium carboxylate used in the invention may be selected from themagnesium salts of the above-mentioned groups of carboxylic acids.

The magnesium carboxylate may be provided in solid (e.g. crystalline)form. Alternatively, the magnesium carboxylate may be in dissolved form,for example as part of a solution or suspension. Such a solution orsuspension comprising dissolved magnesium carboxylate may be aqueous andmay in particular be obtained in a fermentation process. An example of asuspension may for example be a suspension comprising dissolvedmagnesium carboxylate and insoluble biomass, such as a fermentationbroth. In case the magnesium carboxylate is provided in dissolved form,the magnesium carboxylate solution or suspension may have aconcentration of 1-700 g, preferably 100-600 g, more preferably 200-500g magnesium carboxylate per liter solution or suspension.

In case the carboxylate is provided as a solution or suspension, themagnesium carboxylate concentration at which carboxylic acidprecipitation occurs upon acidulation may depend on the HClconcentration. For example, when using a HCl solution with a high HClconcentration (e.g. between 20 and 30 wt. %) to acidify the carboxylate,precipitation of carboxylic acid may occur at relatively low carboxylateconcentrations (e.g. at around or between 1 and 10 wt. %). However, whenusing lower HCl concentration (e.g. between 10 and 20 wt. %), highercarboxylate concentration (e.g. between 10 and 50 wt. %) may be requiredfor precipitation to occur. For practical reasons, the upper limit ofthe magnesium carboxylate concentration in a magnesium carboxylatesolution or suspension is the maximum solubility of magnesiumcarboxylate at a maximal temperature of 75 degrees Celsius. Thisconcentration is typically around 20 wt. % magnesium carboxylate orless, based on the total weight of the solution or suspension. However,it may vary for the specific carboxylate used. Concentrations higherthan 20 wt. % may require the solution to have a temperature of 75° C.or above in order to have the magnesium carboxylate in completelydissolved form, which temperature is bad for the equipment with regardsto the corrosion sensitivities of the materials used in the presence ofHCl.

To yield as much carboxylic acid as possible after acidulation andprecipitation, the carboxylate concentration going into the acidulationis preferably as high as possible. In case the magnesium carboxylate isprovided as a solution, the upper limit of the magnesium carboxylateconcentration is determined by the solubility of the magnesiumcarboxylate and the temperature at which the equipment is stillsufficiently resistant against corrosion due to HCl. In case thecarboxylate is provided as a suspension, the stirrability of thesuspension typically determine the upper limit. In case the carboxylateis provided as a solid cake, the solid liquid separation and resultingadhering water typically determine the upper limit. To support a highcarboxylic acid yield after acidulation and precipitation, the HClconcentration is preferably as high as economically feasible, asintroduction of extra water will dilute the system. The combination ofthe above mentioned input concentrations of carboxylate and HCl mustfavorably result in a situation where MgCl₂ remains in solution and asmuch carboxylic acid as possible precipitates during the precipitationstep. The skilled person will be able to vary the two concentrations toobtain the desired result. For example, good results have been obtainedusing a combination of 15-25 wt. % HCl and a magnesium carboxylateconcentration of 20-50 wt. %.

In case a magnesium carboxylate solution or suspension is obtained froma fermentation process which does not have a sufficiently high magnesiumcarboxylate concentration, the solution may be concentrated, for exampleby evaporation.

In a preferred embodiment of the present invention, the magnesiumcarboxylate is obtained in a fermentation which uses a magnesium-basedbase for neutralisation in order to directly produce magnesiumcarboxylate—in contrast to first conducting fermentation and then addinga base to form magnesium carboxylate- to keep the process as simple aspossible and to prevent using additional processing steps.

The above-mentioned magnesium-based fermentation may also be run atconditions so that the resulting fermentation product is a mixture ofcarboxylic acid acid and magnesium carboxylate which will lead to lesscarboxylate to be acidulated and precipitated.

The method of the invention further comprises an acidulation step,wherein the magnesium carboxylate is acidified with HCl, therebyobtaining a solution comprising carboxylic acid and MgCl₂. The inventorsfound that HCl is preferred as an acidifying agent over other acids,such as H₂SO₄. First, the use of HCl provides for an efficientprecipitation, such as the advantageous salting out effect describedabove. In particular, the presence of MgCl₂ decreases the solubility ofthe carboxylic acid, which results in a more efficient precipitation ofthe acid. Furthermore, the reaction of magnesium carboxylate with HClresults in salt with a relatively high solubility (MgCl₂), in particularcompared to other magnesium salts including MgSO₄ and also compared tomany carboxylic acids. A high solubility of the salt obtained byacidifying is desirable, because as little of this salt as possibleshould precipitate in the precipitation step. The maximum concentrationof carboxylic acid in the solution to be precipitated is therefore inpart determined by the solubility of the salt obtained in theacidulation step. Thus, in case the salt has a high solubility, a highcarboxylic acid concentration can be obtained without precipitation ofthe salt, which results in an efficient precipitation of the carboxylicacid.

Acidulation is typically conducted using an excess of HCl. The excess ispreferably small, such that the MgCl₂ solution obtained afterprecipitation is not highly acidic, which may not be desirable in viewof further processing such a solution. For example, the excess of HClused may be such that the resulting MgCl₂ solution after precipitationhas a pH of 1 or higher, such as a pH of about 1.5. The skilled personknows how to calculate based on reaction stochiometrics the maximalallowable excess for such a pH of 1 or higher. To obtain a sufficientlycomplete acidulation, the resulting MgCl₂ solution preferably has a pHbelow 4, more preferably below 3.

HCl acidulation may for example be conducted by bringing the magnesiumcarboxylate in contact with HCl, for example by bringing the magnesiumcarboxylate (in solid form, suspension or solution) in contact with anaqueous HCl solution or by bringing a magnesium carboxylate solution orsuspension in contact with HCl gas.

If a HCl solution is used in the acidulation step, it preferablycomprises at least 5 wt. %, more preferably at least 10 wt. % and evenmore preferably at least 20 wt. % HCl. Such concentrations are generallysufficient to acidify the magnesium carboxylate. High HCl concentrationsmay be desirable due to the above-mentioned salt out effect. Due to thelow boiling point of HCl and the HCl/H₂O azeotrope, the HClconcentration in a HCl solution will typically not be higher than 40%,in particular when using a HCl solution at atmospheric pressure.Preferably, a HCl concentration is used with a concentration of 15-25wt. % HCl, based on the total weight of the HCl solution. Nevertheless,HCl concentrations of up to 100% may also be employed, in which case aHCl solution is typically used under increased pressure (e.g. aboveatmospheric pressure) and/or low temperatures (e.g. below 20° C.).

In case HCl gas is used, HCl gas may be contacted by bringing it incontact with a carboxylate solution or suspension. In particular, HClgas may be blown through the solution or suspension. In case HCl gas isused, the HCl may originate from a thermal decomposition step, such asfor example described below.

Preferably, acidification is conducted at a temperature of 75° C. orless. At higher temperatures, it becomes uneconomical to adapt equipmentto the harsh conditions. In view of the freezing point of water,acidification is typically conducted at a temperature above 0° C.Temperatures above 20° C. may be preferred to avoid the use of coolingmachines. Temperatures of 40° C. or more, or even 60° C. or more areeven more preferred, because more magnesium carboxylate can be dissolvedat these higher temperatures. The temperature of the magnesiumcarboxylate solution or suspension is typically determined by andcorresponds with the temperature at which the acidification isconducted.

The method of the invention may comprise a concentration step, whereinthe solution obtained after acidulation with HCl is concentrated. Ahigher concentration of carboxylic acid in the solution will increasethe efficiency of the carboxylic acid precipitation. The concentrationstep may be conducted by evaporation. In the concentration step, 10-90%of the total amount of water present in the solution may be removed.However, preferably no MgCl₂ is precipitated as a result of theconcentration. Therefore, the solution obtained after acidulation ispreferably concentrated to a MgCl₂ concentration that is equal or lowerto the saturation point of MgCl₂.

The method of the invention further comprises precipitating thecarboxylic acid from the solution obtained in the acidulation step or,if present, from the solution obtained in the concentration step. Thisstep may be referred to as the (first) precipitation step. Precipitationmay be conducted by any precipitation method known in the art, such asreactive precipitation or by cooling, concentrating, evaporating thesolution to be precipitated or by adding an antisolvent to the solutionto be precipitated.

Precipitation is preferably established by acidifying the magnesiumcarboxylate with HCl. This type of precipitation may be referred to asreactive precipitation. In reactive precipitation, precipitation takesplace during acidulation. Consequently, acidifying the magnesiumcarboxylate and precipitating the thus obtained carboxylic acid areconducted as one step. Accordingly, the method of the invention willcomprise the steps of providing magnesium carboxylate obtainedoptionally in a fermentation process (as described above); andacidifying the magnesium carboxylate with HCl (e.g. an aqueous HClsolution), thereby obtaining a carboxylic acid precipitate and a MgCl₂solution. It is noted that the precipitation step actually results in asuspension with the carboxylic acid precipitate present in the MgCl₂solution.

Reactive precipitation can be conducted by choosing the conditions inthe acidulation step such that immediate precipitation of the carboxylicacid can occur. The skilled person will know how to establish suchconditions. In particular, the magnesium carboxylate concentration maybe chosen such that the acidulation with HCl will result in a carboxylicacid concentration that is higher than the saturation point of thecarboxylic acid. The exact concentration of the carboxylic acid at itssaturation point will vary for the carboxylic acid used.

The precipitation step may also be conducted by cooling the solution tobe precipitated, e.g. the solution formed in the acidulation step, or,if present, the solution obtained in the concentration step. This typeof precipitation may be referred to as cooling precipitation. Thecooling step may require that the solution to be precipitated is firstheated to a temperature at which substantially all MgCl₂ and carboxylicacid are dissolved. The solution to be precipitated may be cooled from atemperature above the nucleation temperature of the carboxylic acid inthe solution to a temperature below the nucleation temperature of thecarboxylic acid in the solution. The nucleation temperature is thehighest temperature at which solids, in particular, precipitate, isformed. This temperature is i.a. dependent on the concentration ofMgCl₂, carboxylic acid and the presence of other components. Therefore,it is not possible to give a single temperature value for the nucleationtemperature. However, in general, the solution to be precipitated iscooled from a temperature of at least 35° C. to a temperature of lessthan 30° C., preferably at least 40° C. to a temperature of less than25° C. Higher temperature differences make it possible to increase theyield of carboxylic acid precipitate. In case of a cooling precipitationthe carboxylic acid concentration prior to cooling is preferably asclose to the solubility as is economically feasible. The carboxylic acidconcentration may be equal or up to 5, preferably up to 10 g/L lowerthan the saturation point of the carboxylic acid.

Furthermore, precipitation may be established by concentrating thesolution comprising the carboxylic acid and MgCl₂, preferably byevaporation. Evaporation of part of the solvent of the solutioncomprising the carboxylic acid and MgCl₂ will result in a higherconcentration of the carboxylic acid and a stronger salting out effect,which enhances precipitation.

Furthermore, precipitation may be established by adding an antisolventto the solution to be precipitated. Examples of antisolvents arealcohols, ethers and ketones.

Preferably, the MgCl₂ solution obtained after precipitation may besubjected to a second and/or further precipitation step, thereby formingadditional carboxylic acid precipitate and a second and/or further MgCl₂solution. The second or further precipitation step may be conducted torecover at least part of the carboxylic acid remaining in the MgCl₂solution obtained in the previous precipitation step. In this case, thisprevious precipitation step of the invention may be referred to as thefirst precipitation step. The MgCl₂ solution obtained in the firstprecipitation of the method may still comprise small amounts ofcarboxylic acid. To recover at least part of this carboxylic acid, asecond precipitation step may be conducted. Such a second precipitationstep may be conducted under similar conditions as the firstprecipitation step, including a concentration step and/or the additionof MgCl₂ conducted prior to the precipitation step.

In a preferred embodiment, the method of the invention comprises a firstprecipitation reaction, which is a reactive precipitation step, afterwhich the MgCl₂ solution obtained in this step is subjected to a coolingand/or evaporation step. The cooling and/or evaporation step are furtherprecipitation steps, wherein additional carboxylic acid is precipitatedand carboxylic acid losses and process yields are thus improved.

Prior to any precipitation step, magnesium chloride may be added to thesolution to be precipitated or to the HCl solution. This solution to beprecipitated may be the solution comprising the magnesium carboxylatesolution (e.g. in case of reactive precipitation) or the solutioncomprising carboxylic acid and magnesium chloride (as obtained in theacidulation step). Such added magnesium chloride may increase thesalting out effect, thereby enhancing the precipitation of carboxylicacid.

Preferably, the method further comprises the steps of

-   -   subjecting the MgCl₂ solution to a thermal decomposition step at        temperatures of at least 300° C., thereby decomposing the MgCl₂        to MgO and HCl; and    -   dissolving the HCl formed in the thermal decomposition step in        water, thereby obtaining a HCl solution; and    -   bringing the MgO in contact with water, thereby obtaining        Mg(OH)₂.

As described above, the advantage of these additional steps is that amethod may be obtained that has no or substantially no salt waste.

Thermal decomposition of chlorides is commonly known from the steelindustry, wherein iron(III)chloride (FeCl₃) is thermally decomposed intoiron(II)chloride (FeCl₂) and chlorine gas (Cl₂). In this field, thermaldecomposition of MgCl₂ to HCl and MgO is also known, for example from GB793,700. Thermal decomposition as described herein may also be suitablyapplied in the method of the invention. Accordingly, thermaldecomposition used in the invention may be conducted by spraying theMgCl₂ solution into contact with a stream of hot gas. The temperature ofthe hot gas is equal to the temperature at which thermal decompositionis conducted, as described below.

The combination of thermal decomposition in an acid/salt separation ofmagnesium carboxylate from a fermentation process has to the applicant'sknowledge not been described earlier. The inventors realised that MgCl₂can be thermally decomposed by pyrohydrolysis at relative lowtemperatures (for example in contrast to CaCl₂, which starts todecompose at about 800° C. or higher). This is advantageous, because theMgO formed will still have a sufficiently high reactivity that it can beeffectively used in for example fermentation.

Suitable apparatuses for conducting thermal decomposition are known inthe art. Thermal decomposition may be conducted using a roaster, forexample a spray roaster or a fluid bed roaster. Such apparatuses can forexample be obtained at SMS Siemag. The use of a spray roaster ispreferred. A spray roaster has low energy costs (also compared to afluid bed roaster), because it requires relatively temperatures (asdescribed below). A spray roaster further produces reactive MgOparticles, which are very suitable for use as a neutralizing agent infermentation.

Preferably, thermal decomposition is conducted at a temperature of aleast 300° C., which is the minimum temperature at which MgCl₂decomposes. Preferably, thermal decomposition is conducted at atemperature of at least 350° C., for example 350-450° C. Due to energycosts, the temperature is preferably below 1000° C., more preferablybelow 800° C. For example, the temperature at which thermaldecomposition is conducted may be 350-600° C. or 300-400° C. Inaddition, using a too high temperature for the thermal decompositionstep is undesirable, because it will reduce the reactivity of the MgOformed, such that it is less suitable for use as a neutralizing agent infermentation.

Thermal decomposition as applied in the method of the invention ispreferably conducted at a pressure of 0.1-10 bar. However, the use ofelevated pressure may be undesirable, because of an increased risk ofcorrosion due to the HCl not being able to condense. Preferably, thermaldecomposition is conducted at atmospheric pressure, in particular whenusing a roaster, to avoid unnecessary energy costs and the need forexpensive high pressure equipment.

Magnesium oxide (MgO) is one of the products of the thermaldecomposition and is typically obtained in the form of a powder. Themagnesium oxide is hydrated with water, e.g. by quenching the MgO withwater, thereby forming a magnesium hydroxide (Mg(OH)₂ suspension. Themagnesium hydroxide suspension is preferably recycled for use in thefermentation process. For example, the Mg(OH)₂ may be used as aneutralizing agent in a fermentation process. In this case, the Mg(OH)₂may first be washed with water to remove chloride ions, typically to acontent less than 1000 ppm. The presence of chloride ions isundesirable, because they may cause corrosion problems when added to afermentation vessel. Since Mg(OH)₂ has a low solubility in water, such awashing step will typically not result in the loss of significantamounts of Mg(OH)₂. Alternatively, the Mg(OH)₂ is first converted tomagnesium carbonate (MgCO₃), which is then used as a neutralizing agentin a fermentation process. A combination of these two steps may also beapplied in which part of the Mg(OH)₂ is washed and re-used and a secondpart is converted into MgCO₃ and then re-used in the process. Part ofthe MgO may even be directly used in the fermentation.

The HCl obtained in the thermal decomposition step may be dissolved inwater, thereby forming an aqueous HCl solution. Preferably, the HClobtained in the thermal decomposition step is recycled by using it inthe acidification step in the method of the invention, for example asHCl gas or as an aqueous HCl solution.

The magnesium carboxylate provided in the method of the invention may beobtained in a fermentation process. In such a fermentation process acarbohydrate source is typically fermented by means of a micro-organismto form a carboxylic acid. Subsequently, a magnesium base is added asneutralising agent during fermentation to provide the magnesium salt ofthe carboxylic acid. Examples of suitable magnesium bases are magnesiumhydroxide (Mg(OH)₂), magnesium carbonate (MgCO₃) and magnesiumbicarbonate (Mg(HCO₃)₂). The advantage of the use of Mg(OH)₂ as a baseis that this compound can be provided by the method of the invention.The use of MgCO₃ may also desirable and can be easily obtained byconverting Mg(OH)₂ obtained in the method of the invention. Furthermore,the use of MgCO₃ or Mg(OH)₂ is desirable, because hydroxide andcarbonate are not expected to have a negative effect on the salting outeffect of the method of the invention (any carbonate left afterneutralising may leave the solution as gaseous CO₂).

The fermentation process may comprise a purification step, wherein themagnesium carboxylate obtained during or after crystallisation iscrystallised from the fermentation broth, which may then be subsequentlydissolved in water to form an aqueous solution, which typically has ahigher concentration of carboxylate than the fermentation broth. Such apurification step may have the advantage that a higher yield can beobtained in the first precipitation step due to the higher concentrationof the magnesium carboxylate.

However, as described above, the magnesium carboxylate preferablyremains in dissolved form when the magnesium base is added as aneutralizing agent. This has the advantage that the magnesiumcarboxylate is pump able and can be directly used in the acidulationstep. Furthermore, the acidulation step is easy to control when themagnesium carboxylate is in dissolved form. In particular, the magnesiumcarboxylate present in the magnesium carboxylate solution or suspensionobtained after adding the magnesium base comprises at least 95 wt. %,preferably at least 99 wt. % of magnesium carboxylate in dissolved form.Small amounts of solids (up to 10 wt. %) of solid matter may not yetlead to the negative effects described above.

The crystallisation may comprise at least one of a concentration step,such as a water evaporation step, a cooling step, a seeding step, aseparation step, a washing step and a re-crystallisation step.Concentration may be performed as a separate step or together withcrystallisation (e.g. evaporative-crystallisation).

The invention is further illustrated by the following examples.

EXAMPLE 1: MAGNESIUM DICARBOXYLATE PREPARATION

Magnesium hydroxide was added to a solution of dicarboxylic acid inwater and heated up to complete dissolution. Four different carboxylicacids were used: adipic acid, fumaric acid, itaconic acid and2,5-furandicarboxylic acid. The amounts of each component are given inTable 1. The resulting dicarboxylate solution was meant to resemble amagnesium dicarboxylate solution obtained in a fermentation process.Although a magnesium dicarboxylate solution obtained in a fermentationprocess generally comprises compounds other than magnesiumdicarbooxylate, such as a relatively large amount of impurities, themagnesium dicarboxylate solution prepared for this example wasconsidered to sufficiently resemble a magnesium dicarboxylate solutionobtained in a fermentation process to show the proof of principle thatthe invention works.

TABLE 1 Magnesium Dicarboxylic Type of dicarboxylic oxide acid wateracid [g] [g] [g] Aclipic acid 47 171 767 Fumaric acid 6.4 18.6 1082Itaconic acid 51 164 745 2,5-furandicarboxylic 2.4 9.5 528 acid

EXAMPLE 2: DICARBOXYLIC ACID PRECIPITATION

A certain amount of an aqueous solution of HCl was added to themagnesium dicarboxylate solution from Example 1, as indicated in Table2. The temperature of the thus obtained mixtures is also given in Table2. The mixture was cooled to 20° C. and a precipitate was formed. Duringcooling, samples were taken of the solution for each 10±1 centigrade.The composition of the samples and the total amount of precipitateformed were determined.

TABLE 2 Magnesium Dicarboxylate HCl Type of dicarboxylic solutionconcentration HCl T, o acid [g] [% (g/g)] [g] [° C.] Aclipic acid 985 37231 80 Fumaric acid 1107 34.4 36 60 Itaconic acid 960 37 249 602,5-furandicarboxylic 541 37 12 50 acid

The samples were taken only from the solution (for sampling, stirrer wasstopped some few seconds, and after crystal settling, sample taken fromthe upper layer). Magnesium and dicarboxylic acid in solution wereanalyzed and expressed as g/g water. The amount of crystal produced wascalculated as difference between the initial dicarboxylic mass and theremaining dicarboxylic mass in solution.

The results are shown in Table 3-Table 6 for adipic acid, fumaric acid,itaconic acid and 2,5 furandicarboxylic acid respectively.

TABLE 3 Aclipic Acid Mg Concentration Amount of Temperatureconcentration in the in the solution precipitate (° C.) solution (wt %)(wt %) formed (g) 81 14.2 2.7 0 70 11.5 2.6 37.1 60 6.1 2.7 104.9 51 3.52.8 134.8 40 2 2.7 151.3 30 1.2 2.8 160.0 20 0.7 2.8 165.3

TABLE 4 Fumaric Acid Mg Amount of Temperature concentration in theconcentration in the precipitate (° C.) solution (wt %) solution (mg/kg)formed (g) 60 1.6 3420 0 50 1.1 3450 5.8 39 0.7 3450 10.4 30 0.5 345012.6 20 0.5 3470 12.6

TABLE 5 Itaconic Acid Mg concentration Amount of Temperatureconcentration in in the precipitate (° C.) the solution (wt %) solution(% [g/g]) formed (g) 60 13.2 2.6 0 50 9.4 2.8 50.7 41 6.0 2.7 92.6 304.1 2.8 114.7 20 2.6 2.4 131.5

TABLE 6 2,5-Furandicarboxylic Amount of Temperature Acid concentrationin precipitate (° C.) the solution (%) Mg Conc (wt %) formed (g) 84 0.523260 6.6 72 0.19 3350 8.4 62 0.38 3330 7.3 53 0.25 2930 8.0 42 0.33 34207.6 32 0.14 3340 8.6 22 0.06 1190 9.1

These findings correspond to a total recovery of over 97% for adipicacid, 72% for fumaric acid, 80% for itaconic acid and 96% for2,5-furandicarboxylic acid.

This example shows that adipic acid, fumaric acid, itaconic acid and2,5-furandicarboxylic acid can be efficiently obtained using the methodof the invention. During precipitation, the majority of the dicarboxylicacid precipitates, while substantially all magnesium ions remain insolution. It can be concluded that acidulation with HCl and subsequentcrystallization results in a very efficient separation of thedicarboxylic acids from the magnesium dicarboxylate solution.

EXAMPLE 3: PRECIPITATION OF CITRIC ACID

In a first experiment with citric acid, 5 g of citric acid was added toa saturated solution of MgCl₂.

In a second experiment with citric acid, 15 g of citric acid was addedto a saturated solution of MgCl₂.

In a third experiment with citric acid, 5 g of magnesium chloride wasadded to a saturated solution of citric acid.

In a fourth experiment with citric acid, 15 g of magnesium chloride wasadded to a saturated solution of citric acid.

In all four experiments, a precipitate was formed. The citric acid andMg content of the precipitate was analysed using HPLC. The results areshown in Table 7.

TABLE 7 Experiment citric acid (wt. %) Mg (wt. %) MgCl₂ (wt. %)* 1 97.40.76 2.99 2 92.7 1.18 4.62 3 93.0 0.14 0.56 4 86.9 0.93 3.65 *The amountof MgCl₂ was calculated based on the Mg concentration found.

This experiment shows that citric acid can be precipitated from amagnesium chloride solution.

EXAMPLE 4: PREPARATION OF SUCCINIC ACID

Magnesium hydroxide (99 g) was added to a solution of 200 g succinicacid in 888 g water at room temperature and heated up to completedissolution (by visual observation). An amount of 333 g aqueous solutionof HCl (37 wt. % wt %) was added to the thus prepared magnesiumsuccinate solution. The temperature of the thus obtained mixture wasinitially 62° C. The mixture was cooled to 20° C. and a precipitate wasformed. During cooling, samples were taken of the solution and theprecipitate of the mixture at 62, 52, 40, 31 and 20° C. The compositionof the samples and the total amount of precipitate formed weredetermined.

The samples were taken only from the solution (for sampling, stirrer wasstopped some few seconds, and after crystal settling, a sample was takenfrom the supernatant). Magnesium and succinic acid in solution wereanalyzed and expressed as g/g water. The amount of crystal produced wascalculated as difference between the initial succinic acid mass and themass of the succinic acid remaining in solution.

The results are shown in Table 8.

TABLE 8 Succinic Acid Mg concentration concentration Amount of in the inthe succinic Temperature solution solution acid (° C.) (wt. %) (wt. %)formed (g) 62 13.13 2.71 0 52 8.20 1.82 82 40 5.00 3.15 130 31 3.40 3.20153 20 2.10 3.19 171

Furthermore, the amount of succinic acid in the 182 g precipitate formedduring the cooling step was determined, which was 94.4% corresponding to172 g. The rest of the precipitate consisted mainly of water (4.4%) andmagnesium chloride. These findings correspond to a total recovery ofsuccinic acid of over 85%.

This example shows that during precipitation, the majority of succinicacid precipitates, while substantially all magnesium ions remain insolution. It can be concluded that acidulation with HCl and subsequentcrystallization results in a very efficient separation of succinic acidfrom the magnesium succinate solution.

EXAMPLE 5: PRECIPITATION AFTER CONCENTRATING

To the magnesium succinate solution as prepared in Example 4 an aqueoussolution of HCl (37 wt. %) was added, thereby obtaining 500 g solutioncomprising 2.1 wt. % succinic acid and 12.6 wt. % MgCl₂ (correspondingto a MgCl₂ concentration of 14.8 g per 100 g water). The solution wasthen concentrated by water evaporation, thereby obtaining 199 g solutioncomprising 5.3 wt. % succinic acid and 31.7 wt. % magnesium chloride(corresponding to a MgCl₂ concentration of 50.2 g per 100 g water, whichis close to the saturation point of MgCl₂ in water, which is 55 g/100 gwater at 20° C.). The initial and final values of the solution aresummarized in Table 9.

TABLE 9 MgCl2 ratio to water concentration (wt %) (mass based) mass (g)MgCl₂ Succinic g/100 gH2O initial 500 12.6 2.1 14.8 final 199 31.7 5.350.2

The solution was then cooled from 115° C. to 20° C. Precipitationstarted at 82° C. and continued until 20° C. The precipitate wasseparated from the solution by filtration using a standard gravityfilter. The composition of the precipitate and the solution is show inTable 10.

TABLE 10 Content Mg⁺² water succinic (%) Cl⁻ (wt. %) (wt. %) (wt. %)Solution 0.22 25.0 6.6 —

The succinic acid present in the filtrate was determined usinghigh-performance liquid chromatography (HPLC) and was 0.22 wt. %.Assuming that all succinic acid not present in the filtrate would bepresent in the precipitate, the value of 0.22 wt. % would correspond toa succinic acid yield in the precipitate of over 90%.

The invention claimed is:
 1. A method for the recovery of a carboxylicacid, comprising: providing an aqueous solution or suspension ofmagnesium carboxylate, wherein the carboxylic acid corresponding withthe carboxylate has a solubility in water at 20° C. of 80 g/100 g wateror less; obtaining solid magnesium carboxylate from the solution orsuspension, dissolving the solid magnesium carboxylate in water tocreate an aqueous solution, and acidifying the obtained aqueous solutionof magnesium carboxylate with a solution of hydrogen chloride (HCl),thereby obtaining a solution comprising carboxylic acid and magnesiumchloride (MgCl₂); optionally a concentration step, wherein the solutioncomprising carboxylic acid and MgCl₂ is concentrated; and, precipitatingcarboxylic acid from the solution comprising carboxylic acid and MgCl₂,thereby obtaining a carboxylic acid precipitate and a MgCl₂ solution. 2.The method according to claim 1, wherein the carboxylic acid has asolubility in water at 20° C. that is lower than that of MgCl₂.
 3. Themethod according to claim 1, wherein the carboxylic acid has asolubility in water of less than 30 g/100 g water.
 4. The method ofclaim 1, wherein the carboxylic acid is selected from the groupconsisting of adipic acid, itaconic acid, 2,5-furandicarboxylic acid,fumaric acid, oxalic acid, maleic acid, glutaric acid, malonic acid, andfatty acids having more than 10 carbon atoms.
 5. The method according toclaim 1, wherein: the aqueous solution or aqueous suspension comprisesat least 10 wt. % magnesium carboxylate, based on the total weight ofthe solution or suspension; and, the solution comprising the carboxylicacid and MgCl₂ comprises at least 5 wt. % of magnesium chloride based onthe total weight of the solution comprising carboxylic acid.
 6. Themethod according to claim 1, wherein magnesium carboxylate is providedin dissolved form, as part of an aqueous solution obtained in afermentation process or as part of an aqueous suspension obtained in afermentation process.
 7. The method according to claim 6, wherein themagnesium carboxylate is provided in dissolved form as part of anaqueous suspension obtained in a fermentation process by neutralizationof carboxylic acid with a magnesium base and wherein, in the aqueoussuspension, at least 95 wt. % of the magnesium carboxylate is indissolved form.
 8. The method according to claim 7, wherein the aqueoussuspension consists of dissolved magnesium carboxylate and insolublebiomass.
 9. The method according to claim 1, wherein the aqueoussolution or suspension of magnesium carboxylate provided in theproviding step is concentrated by evaporation to obtain the solidmagnesium carboxylate.
 10. The method according to claim 9, wherein, inthe obtaining solid magnesium carboxylate from the solution orsuspension step, the concentrated solution or suspension of magnesiumcarboxylate is subjected to solid-liquid separation.
 11. The methodaccording to claim 1, wherein acidifying the magnesium carboxylate andprecipitating the carboxylic acid thus formed are conducted in one step.12. The method according to claim 1, wherein the MgCl₂ solution orconcentrated MgCl₂ solution is subjected to a second precipitation stepto recover at least part of the carboxylic acid remaining in the MgCl₂solution obtained in the first precipitation step.
 13. The methodaccording to claim 12, wherein the second precipitation is conducted bycooling and/or concentrating the MgCl₂ solution.
 14. The methodaccording to claim 12, wherein additional MgCl₂ is added to the MgCl₂solution prior to the second precipitation.
 15. The method according toclaim 1, wherein the solution comprising the carboxylic acid and MgCl₂is concentrated to a carboxylic acid concentration that is equal or upto 10 g/L lower than the saturation point of the carboxylic acid. 16.The method according to claim 1, wherein the solution comprising thecarboxylic acid and MgCl₂ is concentrated to a carboxylic acidconcentration that is equal or up to 5 g/L lower than the saturationpoint of the carboxylic acid.
 17. The method according to claim 1,further comprising: subjecting the MgCl₂ solution to a thermaldecomposition step at temperatures of at least 300° C., therebydecomposing the MgCl₂ to magnesium oxide (MgO) and HCl; dissolving theHCl formed in the thermal decomposition step in water, thereby obtaininga HCl solution; and bringing the MgO in contact with water, therebyobtaining Mg(OH)₂, which Mg(OH)₂ is optionally recycled for use in afermentation process.
 18. The method according to claim 17, whereinthermal decomposition is conducted using a spray roaster.
 19. The methodaccording to claim 17, wherein the thermal decomposition is conducted ata pressure in a range of from 0.1-10 bar.
 20. The method according toclaim 17, wherein the thermal decomposition is conducted at atemperature ranging from 300-450° C.
 21. The method according to claim17, wherein Mg(OH)₂ is converted to MgCO₃, which is then used as aneutralizing agent in a fermentation process.
 22. The method accordingto claim 17, wherein thermal decomposition is conducted by spraying theMgCl₂ solution into contact with a stream of hot gas.
 23. The methodaccording to claim 1, further comprising subjecting the MgCl₂ solutionto a thermal decomposition step at temperatures of at least 300° C.,thereby decomposing the MgCl₂ to magnesium oxide (MgO) and HCl.
 24. Themethod according to claim 23, wherein the HCl obtained in the thermaldecomposition step is used in the acidifying step either as HCl gas oras an aqueous HCl solution, the solution being obtained throughdissolving the HCl formed in the thermal decomposition step in water.25. The method according to claim 23, wherein at least a part of the MgOis recycled for use in a fermentation process.
 26. The method accordingto claim 23, further comprising bringing at least a part of the MgO incontact with water, thereby obtaining Mg(OH)₂.
 27. The method accordingto claim 26, wherein at least a part of the Mg(OH)₂ is recycled for usein a fermentation process.
 28. The method according to claim 23, furthercomprising bringing at least a part of the MgO in contact with water,thereby obtaining Mg(OH)₂; and, converting at least a part of theMg(OH)₂ to MgCO₃, which is then used as a neutralizing agent in afermentation process.
 29. The method according to claim 6, wherein theaqueous solution obtained in the fermentation process or suspensionobtained in the fermentation process further comprises the carboxylicacid.
 30. The method according to claim 6, wherein the magnesiumcarboxylate is provided in dissolved form as part of an aqueoussuspension obtained in a fermentation process by neutralization ofcarboxylic acid with a magnesium base, and wherein the aqueoussuspension comprises up to 10 wt. % solids.